docCollection ease blocking.

[sijuv]

https://issues.apache.org/jira/browse/SOLR-XXXXX

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[dsmiley]

Please tell us more about this without us having to read the code to guess

[sijuv]

@dsmiley still working on it. The observation is that under high load all application threads stall at the getDocCollection esp if all the requests are for the same collection. This PR (still in works) is to modify the behavior so that only one thread is blocked while others read the stale value.